This invention relates to an improved proportioning pump and more particularly to such a pump which is used in a blood dialysis system.
The proportioning of water with dialysate concentrate with a long-term steady state accuracy requires a highly accurate proportioning device. Those proportioning techniques which are dependent on a minimal number of physical properties remaining constant are inherently more stable and capable of higher accuracy. Physical properties which can vary include water and concentrate density, viscosity, bubbles of air, flow coefficients of orifices, constant fluid leakage characteristics and so on.
One type of proportioning pump which has been successfully used includes motor driven positive displacement cylinders. An example of such a commercially available pump is the mRoy proportioning pump available from the Milton Roy Company, St. Petersburg, Florida. U.S. Pat. No. 3,441,136 -- Serfass shows the use of such a pump in a blood dialysis system.
Feedback controlled proportioning systems automatically attempt to adjust their control action to offset one or more changing physical properties. U.S. Pat. No. 3,508,656 -- Serfass et al shows a mixing pump which is feedback controlled from a conductivity cell.
Pumps having positive displacement cylinders with reciprocating pistons are driven by water pressure. U.S. Pat. Nos. 3,406,826 and 3,598,727 show dialysis systems having such pumps. These pumps have separate cylinders with an exposed piston rod connecting the cylinders. The exposed rod has seals which can leak creating build-up of salt encrustation. These pumps have valves which are driven by an exposed mechanical linkage between the reciprocating piston and the values. This exposes the operator to possible injury from the moving linkage parts. The mechanical linkage draws energy from the limited water pressure energy available for operation of the pump resulting in slower operation of the pump than would otherwise be possible. Also, the valve is slow to operate and causes a significant interruption of flow.